1. Field of the Invention
The present invention relates to a laser transmitter device, and more particularly to a technique for locking the wavelength of light emitted from a laser light source for optical communication at a predetermined wavelength.
2. Description of the Related Art
As a method of increasing the capacity of optical communication, a DWDM (Dense Wavelength-Division Multiplexing) method is known. Since DWDM optical communication has advantages in that, e.g., it can effectively utilize properties of optical fibers in terms of broad band and large capacity, it is considered as a promising method, and research and development thereof proceed. Light sources used in DWDM optical communication are required to output laser light with a plurality of different wavelengths. The wavelengths have to be set at wavelengths decided for respective channels based on a recommendation from ITU-T (International Telecommunication Union—Telecommunication Standardization Section).
For example, in the case of a conventional DWDM optical communication system, which performs wavelength multiplex of 32 wavelengths, it is provided with 32 semiconductor lasers for oscillating laser light with wavelengths different from each other, or 32 wavelength-tunable lasers, which can change their oscillation wavelengths by adjusting the laser temperature or driving current. DFB (Distributed Feedback) lasers or DBR (Distributed Bragg Reflector) lasers are used as the wavelength-tunable lasers.
Particularly, where wavelength-tunable lasers are used, it is possible to reduce the number of kinds of semiconductor lasers used as ordinary-use light sources and preparatory light sources in a light source system for DWDM optical communication. For example, in a DWDM optical communication system with 32 wavelengths, if one semiconductor laser is used for one wavelength, it is necessary to use 64 semiconductor lasers in total, consisting of 32 ordinary-use light sources and 32 preparatory light sources. On the other hand, if wavelength-tunable semiconductor lasers each being capable of oscillating 8 wavelengths are used, only 4 kinds of lasers suffice the preparatory light sources.
Each of these semiconductor lasers has a diffraction grating whose pitch or the like is designed to oscillate single mode laser light with a predetermined wavelength in a steady state, but it is not always locked at the predetermined wavelength. For example, at the startup, the semiconductor laser normally does not oscillate at the predetermined wavelength. Even in the steady state, there are certain fluctuations. Accordingly, any wavelength-tunable laser capable of oscillating a plurality of wavelengths suffers the phenomena described above, and thus it is necessary to stabilize it at an aimed one of predetermined wavelengths.
As described above, in DWDM optical communication, the oscillation wavelength of each semiconductor laser has to be set at a predetermined channel wavelength with high accuracy. Since the oscillation wavelength of each semiconductor laser varies with time, a wavelength monitor function is added to keep the oscillation wavelength constant.
FIG. 32 is a view showing the internal structure of a CW (continuous-wave) laser transmitter device (module) with a conventional wavelength monitor function built therein. This device 10 includes a DFB laser (light source) 11, an FP (Fabry-Perot) filter 16, monitor PDs (Photo Diode) 17 and 34, and so forth. These members are mounted, through a carrier 18, on a Peltier element 19 for temperature control. Light emitted forward from the DFB laser 11 focuses on an optical fiber 14 through lenses 12 and 13.
Light emitted backward from the DFB laser 11 in the reverse direction relative to the optical fiber 14 passes through a lens 15 and is divided into two parts by a beam splitter 32. One of the light outputs divided by the beam splitter 32 passes through an FP filter 16 and is incident on a monitor PD 17, which outputs an electric current in accordance with reception light intensity. The FP filter 16 is designed to have an FSR (Free Spectral Range) equal to the channel intervals in DWDM optical communication. The other of the light outputs divided by the beam splitter 32 is directly incident on a monitor PD 34.
For example, the DFB laser (wavelength-tunable laser) 11 is assigned to 4 channels ch1 to ch4 with wavelength intervals of 100 GHz (about 0.8 nm) based on an ITU-T recommendation. The DFB laser 11 oscillates light with a wavelength corresponding to one of 4 channels, while its temperature being controlled by the Peltier element 19.
The output current of the monitor PD 17 is used by a controller (wavelength lock control circuit (AFC)) 21 to adjust the temperature of the Peltier element 19, so as to control the oscillation wavelength of the DFB laser 11 to be constant. On the other hand, the output current of the monitor PD 34 is used by a controller (output control circuit (APC)) 36 to adjust the current fed to the DFB laser 11, so as to control the intensity of the optical output of the DFB laser 11 to be constant. Each of the controllers 21 and 36 is formed of, e.g., an MPU (Microprocessor Unit) for control.
FIG. 33 is a graph showing the wavelength dependency of the output current Ipd of the monitor PD 17 in the laser transmitter device shown in FIG. 32. In FIG. 33, the horizontal axis denotes the DFB laser oscillation wavelength, and the vertical axis denotes the monitor output current. The output current Ipd periodically changes with the wavelength, in the same cycle as the channel intervals, because it reflects the transmission characteristic of the FP filter 16. In this conventional example, when the output current Ipd takes on a target value X0, the oscillation wavelength coincides with one of the channel wavelengths.
FIG. 34 is a view for explaining a wavelength control operation in the laser transmitter device shown in FIG. 32. In this wavelength control operation, the temperature of the DFB laser 11 is controlled by the Peltier element 19 so that the output current Ipd takes on the target value X0. The controller 21 receives the output current Ipd, and, when the output current Ipd is larger than the target value X0 (a point Ja in FIG. 34), it controls the oscillation wavelength to be longer. Conversely, when the output current Ipd is smaller than the target value X0 (a point Jb in FIG. 34), it controls the oscillation wavelength to be shorter. As a consequence, the output current Ipd is always kept at the target value X0, i.e., the oscillation wavelength of the DFB laser 11 is stabilized at an aimed channel wavelength.
FIG. 35 is a graph showing the wavelength dependency of the output current Ipd of the monitor PD 17, in a case where the increase and decrease in a control parameter for the oscillation wavelength (such as temperature) is controlled in reverse between the even-numbered channels and odd-numbered channels, so that the size of the channel intervals can be halved. In this case, for example, the DFB laser 11 is assigned to 8 channels ch0 to ch7 with wavelength intervals of 50 GHz (about 0.4 nm) based on an ITU-T recommendation. The FP filter 16 is designed to have an FSR equal to two times the channel intervals in DWDM optical communication.
In the control method shown in the FIG. 35, when the oscillation wavelength of the DFB laser 11 is tuned to (stabilized at) an even-numbered channel, the controller 21 performs a wavelength control operation, as follows. Specifically, when the DFB laser 11 starts up, the controller 21 receives an instruction to tune the wavelength at the even-numbered channel. If the output current Ipd is larger than the target value X0, the controller 21 controls the oscillation wavelength to be longer. If the output current Ipd is smaller than the target value X0, the controller 21 controls the oscillation wavelength to be shorter.
In the control method shown in the FIG. 35, when the oscillation wavelength of the DFB laser 11 is tuned to (stabilized at) an odd-numbered channel, the controller 21 performs a wavelength control operation in reverse to that described above, as follows. Specifically, when the DFB laser 11 starts up, the controller 21 receives an instruction to tune the wavelength at the odd-numbered channel. If the output current Ipd is larger than the target value X0, the controller 21 controls the oscillation wavelength to be shorter. If the output current Ipd is smaller than the target value X0, the controller 21 controls the oscillation wavelength to be longer.
However, in either case of using the control method shown in FIG. 33 or 35, when the DFB laser 11 starts up, it does not necessarily oscillate at a predetermined wavelength. Besides, the controller 21 controls the oscillation wavelength on the basis only of comparison in the magnitude relationship between the monitor current Ipd and target value X0. Furthermore, the transmission characteristic of the FP filter (such as an etalon) 16 relative to the wavelength shifts with the temperature change of the filter 16. As a consequence, the control of tuning the oscillation wavelength of the DFB laser 11 to a predetermined wavelength suffers problems described later.
Under the circumstances, there are demands for a laser transmitter device, which can reliably tune the oscillation wavelength of a laser light source for optical communication to a predetermined wavelength.